Re: [PATCH v6 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()

[Lance Yang]

Hi David,

Thanks for taking time to review!

On Wed, Jun 5, 2024 at 8:50 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 21.05.24 06:02, Lance Yang wrote:
> > When the user no longer requires the pages, they would use
> > madvise(MADV_FREE) to mark the pages as lazy free. Subsequently, they
> > typically would not re-write to that memory again.
> >
> > During memory reclaim, if we detect that the large folio and its PMD are
> > both still marked as clean and there are no unexpected references
> > (such as GUP), so we can just discard the memory lazily, improving the
> > efficiency of memory reclamation in this case.
> >
> > On an Intel i5 CPU, reclaiming 1GiB of lazyfree THPs using
> > mem_cgroup_force_empty() results in the following runtimes in seconds
> > (shorter is better):
> >
> > --------------------------------------------
> > |     Old       |      New       |  Change  |
> > --------------------------------------------
> > |   0.683426    |    0.049197    |  -92.80% |
> > --------------------------------------------
> >
> > Suggested-by: Zi Yan <ziy@xxxxxxxxxx>
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >   include/linux/huge_mm.h |  9 +++++
> >   mm/huge_memory.c        | 80 +++++++++++++++++++++++++++++++++++++++++
> >   mm/rmap.c               | 41 ++++++++++++++-------
> >   3 files changed, 117 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 9fcb0b0b6ed1..cfd7ec2b6d0a 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -411,6 +411,8 @@ static inline bool thp_migration_supported(void)
> >
> >   void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> >                          pmd_t *pmd, bool freeze, struct folio *folio);
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > +                        pmd_t *pmdp, struct folio *folio);
> >
> >   #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> >
> > @@ -478,6 +480,13 @@ static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
> >                                        unsigned long address, pmd_t *pmd,
> >                                        bool freeze, struct folio *folio) {}
> >
> > +static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma,
> > +                                      unsigned long addr, pmd_t *pmdp,
> > +                                      struct folio *folio)
> > +{
> > +     return false;
> > +}
> > +
> >   #define split_huge_pud(__vma, __pmd, __address)     \
> >       do { } while (0)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 425272c6c50b..4793ffa912ca 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2687,6 +2687,86 @@ static void unmap_folio(struct folio *folio)
> >       try_to_unmap_flush();
> >   }
> >
> > +static bool __discard_trans_pmd_locked(struct vm_area_struct *vma,
>
> Can we move towards folio terminology?
>
> __discard_anon_folio_pmd_locked() or sth like that?

Nice, it's much clearer!

>
> > +                                    unsigned long addr, pmd_t *pmdp,
> > +                                    struct folio *folio)
> > +{
> > +     VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio);
> > +     VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
> > +
> > +     struct mm_struct *mm = vma->vm_mm;
> > +     int ref_count, map_count;
> > +     pmd_t orig_pmd = *pmdp;
> > +     struct page *page;
> > +
> > +     if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
> > +             return false;
> > +
> > +     page = pmd_page(orig_pmd);
> > +     if (unlikely(page_folio(page) != folio))
> > +             return false;
> > +
> > +     if (folio_test_dirty(folio) || pmd_dirty(orig_pmd)) {
> > +             folio_set_swapbacked(folio);
> > +             return false;
> > +     }
> > +
> > +     orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp);
> > +
> > +     /*
> > +      * Syncing against concurrent GUP-fast:
> > +      * - clear PMD; barrier; read refcount
> > +      * - inc refcount; barrier; read PMD
> > +      */
> > +     smp_mb();
> > +
> > +     ref_count = folio_ref_count(folio);
> > +     map_count = folio_mapcount(folio);
> > +
> > +     /*
> > +      * Order reads for folio refcount and dirty flag
> > +      * (see comments in __remove_mapping()).
> > +      */
> > +     smp_rmb();
> > +
> > +     /*
> > +      * If the folio or its PMD is redirtied at this point, or if there
> > +      * are unexpected references, we will give up to discard this folio
> > +      * and remap it.
> > +      *
> > +      * The only folio refs must be one from isolation plus the rmap(s).
> > +      */
> > +     if (folio_test_dirty(folio) || pmd_dirty(orig_pmd))
> > +             folio_set_swapbacked(folio);
> > +
> > +     if (folio_test_swapbacked(folio) || ref_count != map_count + 1) {
> > +             set_pmd_at(mm, addr, pmdp, orig_pmd);
> > +             return false;
> > +     }
> > +
> > +     folio_remove_rmap_pmd(folio, page, vma);
> > +     zap_deposited_table(mm, pmdp);
> > +     add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> > +     if (vma->vm_flags & VM_LOCKED)
> > +             mlock_drain_local();
> > +     folio_put(folio);
> > +
> > +     return true;
> > +}
> > +
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > +                        pmd_t *pmdp, struct folio *folio)
> > +{
> > +     VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> > +     VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> > +     VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
> > +
> > +     if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> > +             return __discard_trans_pmd_locked(vma, addr, pmdp, folio);
> > +
> > +     return false;
> > +}
> > +
> >   static void remap_page(struct folio *folio, unsigned long nr)
> >   {
> >       int i = 0;
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 08a93347f283..249d6e305bec 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1630,6 +1630,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >       enum ttu_flags flags = (enum ttu_flags)(long)arg;
> >       unsigned long pfn;
> >       unsigned long hsz = 0;
> > +     bool pmd_mapped = false;
> >
> >       /*
> >        * When racing against e.g. zap_pte_range() on another cpu,
> > @@ -1677,18 +1678,26 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >                       goto walk_done_err;
> >               }
> >
> > -             if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > -                     /*
> > -                      * We temporarily have to drop the PTL and start once
> > -                      * again from that now-PTE-mapped page table.
> > -                      */
> > -                     split_huge_pmd_locked(vma, range.start, pvmw.pmd, false,
> > -                                           folio);
> > -                     pvmw.pmd = NULL;
> > -                     spin_unlock(pvmw.ptl);
> > -                     pvmw.ptl = NULL;
> > -                     flags &= ~TTU_SPLIT_HUGE_PMD;
> > -                     continue;
> > +             if (!pvmw.pte) {
> > +                     pmd_mapped = true;
> > +                     if (unmap_huge_pmd_locked(vma, range.start, pvmw.pmd,
> > +                                               folio))
> > +                             goto walk_done;
> > +
> > +                     if (flags & TTU_SPLIT_HUGE_PMD) {
> > +                             /*
> > +                              * We temporarily have to drop the PTL and start
> > +                              * once again from that now-PTE-mapped page
> > +                              * table.
> > +                              */
> > +                             split_huge_pmd_locked(vma, range.start,
> > +                                                   pvmw.pmd, false, folio);
> > +                             pvmw.pmd = NULL;
> > +                             spin_unlock(pvmw.ptl);
> > +                             pvmw.ptl = NULL;
> > +                             flags &= ~TTU_SPLIT_HUGE_PMD;
> > +                             continue;
> > +                     }
> >               }
> >
> >               /* Unexpected PMD-mapped THP? */
> > @@ -1816,7 +1825,13 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >                        */
> >                       if (unlikely(folio_test_swapbacked(folio) !=
> >                                       folio_test_swapcache(folio))) {
> > -                             WARN_ON_ONCE(1);
> > +                             /*
> > +                              * unmap_huge_pmd_locked() will unmark a
> > +                              * PMD-mapped folio as lazyfree if the folio or
> > +                              * its PMD was redirtied.
> > +                              */
> > +                             if (!pmd_mapped)
>
> Isn't that simply "pvmw.pte" ?
>
> Also,
>
> WARN_ON_ONCE(!pmd_mapped);

Good catch! I'll adjust as you suggested.

Thanks again for the review!
Lance

>
> > +                                     WARN_ON_ONCE(1);
> >                               goto walk_done_err;
> >                       }
> >
>
> --
> Cheers,
>
> David / dhildenb
>